The present invention relates to a radio receiver, in particular but not exclusively, to a gain compensation loop for a quadrature receiver.
The current trend in receiver technology is to reduce weight, volume, power consumption and cost. This is particularly important for receivers in portable apparatus such as radio telephones. This has resulted in receiver architecture designs in which there are no or few discrete radio frequency (RF) and intermediate frequency (IF) filters in the receiver front end.
An example of a receiver architecture having few discrete RF and IF filters is a single conversion low-IF architecture for a quadrature receiver. Single conversion low-IF architectures typically produce an image signal which is very close to the wanted signal. Such image signals are termed "in-band" image signals, and may be filtered out using a single sideband filter. However, a portion of the image signal appears at the wanted signal frequency as cross-talk if there is an imbalance between the phase and/or gain of respective quadrature signals. It is desirable for such cross-talk to be reduced or rejected. Typically, a quadrature receiver front end can only achieve about 30dB of image-to-signal cross-talk rejection, which is often insufficient for many applications such as radio telephones.
A solution to the problem of cross-talk is to use a double-quadrature mixer architecture. However, such an architecture requires 90.degree. phase shifts on both the local oscillator and RF ports coupled to four mixers. Providing at least one of the ports (e.g. the local oscillator) is phase and amplitude balanced, any imbalance at the other port (e.g. the RF port) results in a spurious product at a frequency given by the sum of the RF and local oscillator frequencies. This can be easily filtered out using an RF bandpass filter before the mixers.
A drawback of the above approach is that four mixers are required resulting in relatively high power consumption. Additionally, a relatively bulky RF 90.degree. hybrid coupler is also required together with quadrature balance on the local oscillator port.